a) Field of the Invention
By single-channel and multi-channel liquid handling devices is meant in the following both dispensing devices and pipetting devices with one or more dispensing channels by which a liquid can be dispensed simultaneously, particularly in the microliter range and nanoliter range. Broadly speaking, this also includes devices and arrangements which dispense dosed liquid in the processing and manufacturing industries.
b) Description of the Related Art
Devices of the type mentioned above are used particularly for analyzing liquids, e.g., for medical diagnosis and for searching pharmaceutical active substances, where the tendency is toward increasingly smaller measurement volumes which can be handled simultaneously in increasing quantities.
It is required for reproducible analysis results that the individual dispensing volumes do not deviate from a predetermined reference value beyond a predetermined tolerance range and, much more importantly, that they do not deviate from one another beyond a predetermined tolerance range.
In the prior art, in order to maintain these tolerances, the passive components determining the dispensing volumes are correspondingly selected and paired so that identical volumes (within the predetermined tolerance range) are dispensed under identical control parameters.
In addition, or alternatively, by changing the dispensing parameters identical dispensing volumes can be achieved by calibrating the dispensing channels (matching the individual values to a reference value) or equilibrating the dispensing channels (matching the individual values of the channels to one another).
Volume measurement, as is required for the calibration and/or equilibration of liquid handling devices, can be achieved principally by measurement of volume, mass or flow. The predominant principle of measurement for measuring volumes in cells, microtitration plates, or the like optical transparent devices is photometric measurement, whereas gravimetry and calorimetry are predominantly used for determining mass and flow, respectively. An auxiliary liquid with correspondingly suitable optical characteristics is often used on the assumption that the original liquid behaves in like manner with respect to its fluid characteristics.
In photometric methods, a photometric equivalent (reader factor) between the photometric measurement (absorbance, fluorescence) and the volumes of liquid must always be furnished as a reference for accuracy. This is usually carried out using a laboratory scale. For this purpose, the density of the liquid must also be known and may not vary. This value is dependent upon the photometric characteristics of the liquid (dye solution) and the geometry of the vessel (fill level, meniscus, vessel opening, vessel material, etc.). This equivalent must be recalculated whenever the type of plate or vessel is changed or when using a different dye solution (necessary for measuring a different volume).
In a process according to the invention, this can be dispensed entirely because calibration and/or equilibration can be carried out with the liquid used in subsequent operations, while the photometric characteristics of the liquid and measurement chambers have no relevance.
In photometric methods, special precautions must be taken with respect to evaporation because of the required mixing times (diffusion time: 30-60 minutes). When microtiter plates are used as vessels, various methods are employed for reducing evaporation. Masking with a foil (seal) and an ambient atmosphere saturated by the corresponding liquid are known.
On the other hand, the measuring time in a process according to the invention is shorter because no mixing time is required and, therefore, evaporation has less influence. Naturally, a small vessel opening and a saturated ambient atmosphere are also advantageous.
Evaporation can be reduced by means of an advantageous shaping of the vessels.
The influence of surface effects is increased by reducing the volumes and, therefore, the geometric dimensioning of the vessels in which the volumes are to be measured. In this connection, the formation of menisci, acting capillary forces and the wetting behavior must be taken into account. With very small volumes, evaporation becomes a critical factor. As measurement volumes become increasingly smaller in the known measurement methods, measurement errors become larger as a result of these surface effects so that the known measurement methods appear to be poorly suited to calibration and/or equilibration of liquid handling devices dispensing very small volumes.